Acceleration of GRB outflows by Poynting flux dissipation

Abstract We study magnetically powered relativistic outflows in which
a part of the magnetic energy is dissipated internally by
reconnection. For GRB parameters, and assuming that the
reconnection speed scales with the Alfvén speed, significant
dissipation can take place both inside and outside the photosphere
of the flow. The process leads to a steady increase of the flow
Lorentz factor with radius. With an analytic model we show how the
efficiency of this process depends on GRB parameters. Estimates are
given for the thermal and non-thermal radiation expected to be
emitted from the photosphere and the optically thin part of the flow
respectively. A critical parameter of the model is the ratio of Poynting flux to
kinetic energy flux at some initial radius of the flow. For a large
value (
100) the non-thermal radiation dominates over the
thermal component. If the ratio is small (
40) only prompt
thermal emission is expected which can be identified with X-ray
flashes.